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Vlad

Tue Mar 19 2013, 02:19PM

Registered Member #9711 Joined: Sat Jan 19 2013, 03:27PM
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(fixed)

klugesmith

Tue Mar 19 2013, 07:02PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
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Posts: 1716

Using a DVM is asking for trouble, as its range switches try to handle a current source with more than 10,000 volts of compliance.
I have destroyed more DVM's while trying to measure current in high voltage circuits,
than all other abuses put together. Fortunately not many, and mostly the $5 flea market kind.

Short answer:
use a hard-wired low-ohm current sense resistor of adequate power, and probe that only with voltage-mode meters.
Why not start with 1 ohm?

Don't forget the radiation hazard around the tube head,
and the high voltage hazard on the "current" wire if sense resistor is missing, broken, or burned out.

Vlad

Tue Mar 19 2013, 08:17PM

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klugesmith

Tue Mar 19 2013, 08:34PM

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Yup.
I would have suggested starting with 10 or 100 or 1000 ohms, whichever is handy,
except for your report of one time seeing more than 200 mA.

[edit] Thank you for the detailed list of external connections.
I know only two ways to control the emission current:
a) filament temperature and
b) value of grid bias resistor, if the cathode's focusing cup is accessible through a separate terminal of the Coolidge tube.

I bet the dental head connections don't involve b) because the grid bias voltage is at one end of the full high voltage source.

That leaves a), which makes me urge caution. Emission current is -very- sensitive to filament voltage.
At the place where an external resistor is called for, zero ohms might be appropriate only for very low mains voltage.
(do you know what range of values are suggested?)
At normal mains voltage, zero ohms might overheat the filament enough to greatly reduce its lifetime.
Might also result in very high emission current, which could rapidly overheat the tube anode and HV transformer windings.

[edit2] I bet the HV transformer is designed to run close to saturation, like a MOT. Don't be surprised if the primary current exceeds 5 amperes,
even with no filament power and no current in the HV circuit. Does the primary circuit include a resistor and diode in parallel, either within the head or in recommended external circuit?

Vlad

Tue Mar 19 2013, 10:57PM

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klugesmith

Tue Mar 19 2013, 11:41PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Vlad wrote ...
I have an AC analog 0-10 mA meter on the way that I hope will solve the meter problem.

I think a DC analog meter would be more appropriate.
If the x-ray tube is self-rectifying, then the current in HV circuit comes in non-sinusoidal pulses, and is zero in negative half-cycles.
But even if the tube were fed from a full-wave rectifier, or from a DC power supply, that nominal 7 mA value is always the -average- anode current. A DC parameter, supposedly easy to measure.

DC analog and digital meters are very good at measuring the average value of waveforms like that.
AC analog meters are generally calibrated to read the RMS value of sinusoids, and some are sensitive only to current in one direction.

You can check yours, in the actual circuit, by putting it in series with the (sense resistor and DC voltmeter) solution.
It may read correctly within +/- 10%. If it reads too high, you can fix that with an external shunt.

Good luck, and play safely.
The other day I was looking at the oldest archives of this forum.
Found a thread about x-rays, that was immediately closed by a moderator because of the radiation hazard.

Vlad

Wed Mar 20 2013, 12:05AM

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Vlad

Wed Mar 20 2013, 05:07PM

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Steve Conner

Thu Mar 21 2013, 10:09AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
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Maybe the secondary current is really very high and chaotic because the head is arcing internally?

Dr. Dark Current

Thu Mar 21 2013, 10:30AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Excuse my ignorance for not reading the whole thread, but maybe the HV supply is switch mode? In which case, interference could mess up digital meters.

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